With the progress of the information-oriented society, communication networks have come to play an important role as the nervous system in a wide variety of social and economic activities and reliability on them in particular is regarded as important. Reliability on the communications networks is very important also in the aspect of facility control of public communications. When, for example, a local control center introduces a LAN having topology of broadcasting arrangement such as a bus type LAN and a star type LAN in a territory under its control, it is especially important that the communications are continuously secured. It is also important to keep down the initial cost when a LAN facility is introduced and the running cost such as repair and maintenance for operating the facility.
Conventionally, when a local control center introduces a LAN with topology of a broadcasting arrangement such as a bus type LAN and a star type LAN as control media within the territory under its control, it is generally practiced that a plurality of offices or stations are connected to one LAN link path so as to be controlled by the control center. In such an arrangement, however, since there is used only one link path, if there occurs a problem such as breaking of the LAN link at a portion thereof and damage to a joint portion between the LAN link path and a station such as a connector or a driver/receiver, the communications are suspended until the damaged portion is restored to the former condition, and thereby, the reliability on the maintenance service of the control center to keep continued communications and render centralized control is lowered.
Accordingly, there is considered a link changeover control system of a LAN of a broad casting arrangement, as shown in FIG. 1, in which there are provided multiplexed link paths 1 formed of an active LAN link path 2 and a plurality of spare LAN link paths 3a , . . . , 3n with a plurality of node stations 4a, 4b , . . . , 4n connected thereto. This system, as shown in FIG. 1, includes a plurality of LAN medium access control devices (hereinafter referred to as "MAC") 5a, 5b, . . . , 5n for the node stations 4a, 4b, . . . , 4n, connected with the LAN link paths 2, 3a, . . . , 3n, and CPU modules 7 connected to the MAC 5a, 5b, . . . , 5n through CPU bus lines 6 for executing control of logical connections in the LAN.
According to the described arrangement, when, for example, the active LAN link path 2 is broken, or when the MAC 5a connected with the active LAN link path 2 gets out of order, a changeover is made so that the MAC 5b, . . . , 5n connected with another LAN link path 3a, . . . , 3n is put into operation under the control of the CPU module 7 so that the communication can be kept on.
By means of such an arrangement, service for keeping up the communication can be rendered. The arrangement is equally effective in the area where reliable service is required of a general purpose LAN facility.
In a link changeover control system of a LAN of a broadcasting arrangement as described above, it is required that MAC 5a, 5b, . . . , 5n corresponding in number to the multiplexed LAN link paths 2, 3a, . . . , 3n are used, and hence, there arises a difficulty that the initial cost at the introduction of the facility becomes high and the running cost increases for repair and maintenance according as the number of component parts increases.